Porous membranes have recently been utilized in various fields, including not only for treatment of water (e.g. water purification, waste water treatment), medical usage (e.g. blood purification), and the food industry, but also for use as charged membranes, battery separators, and fuel cells.
For example, porous membranes are used in the field of water treatment such as water purification and waste water treatment for substitution for conventional purposes, including sand filtration and coagulative precipitation, and improvement in the quality of treated water. Since a large quantity of water is treated in the field of water treatment, porous membranes are required to have excellent water permeability. Porous membranes with excellent water permeability enable to reduce the area of the membranes, which in turn reduces the size of water purification apparatuses. Accordingly, facility costs can be reduced.
Moreover, porous membranes are required to have chemical resistance because the membranes may be washed with an alkaline solution in chemical cleaning. Studies have been made on porous membranes that contain a fluoropolymer (e.g. polyvinylidene fluoride resin) as a material with high chemical resistance (see, for example, Patent Literatures 1 to 14). However, conventional porous membranes have insufficient water permeability, and thus need improvement.